 | Bauknecht A. Characterization of blade tip vortices on large-scale rotors: Diss. … Dr.-Ing. / Deutsches Zentrum für Luft- und Raumfahrt, Institut für Aerodynamik und Strömungstechnik, Göttingen. - Köln: DLR, 2016. - xxii, 144 p.: ill., tab. - (Forschungsbericht; 2016-06). - Res. also Germ. - Bibliogr.: p.133-144. - ISSN 1434-8454
Шифр: (Pr 1120/2016-06) 02
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Contents ...................................................... vii
List of Figures ................................................ xi
List of Tables ................................................. xv
Nomenclature ................................................. xvii
1 Introduction ............................................... 1
2 Blade Tip Vortices ......................................... 3
2.1 Basic vortex theory ........................................ 3
2.1.1 Vortex generation on a three-dimensional wing ....... 4
2.1.2 Adverse effects of tip vortices ..................... 5
2.1.3 Vortex properties ................................... 5
2.1.4 Vortex models ....................................... 7
2.1.5 Vortex decay and breakdown ......................... 10
2.2 Rotor blade tip vortices .................................. 11
2.2.1 Blade-vortex interactions .......................... 12
2.2.2 Measures against BVI ............................... 13
3 STAR hover test ........................................... 17
3.1 Measurement Techniques .................................... 18
3.1.1 Particle image velocimetry ......................... 18
3.1.2 Seeding particle behavior .......................... 19
3.1.3 Stereoscopic particle image velocimetry ............ 22
3.1.4 PIV on rotors and its predecessors ................. 25
3.1.5 Background-oriented schlieren (BOS) technique ...... 27
3.1.6 Laser speckle-illuminated BOS ...................... 29
3.2 STAR hover test setup ..................................... 30
3.2.1 Hover test facility and rotor test rig ............. 30
3.2.2 Active twist actuation ............................. 32
3.2.3 High-speed PIV setup ............................... 35
3.2.4 High-speed BOS setup ............................... 38
3.2.5 Test sequence ...................................... 41
3.2.6 Measurement procedure and test cases ............... 42
3.3 Data Processing .......................................... 43
3.3.1 Evaluation of PIV and BOS raw data ................. 43
3.3.2 PIV interrogation window size and overlap study .... 44
3.3.3 Particle void detection ............................ 46
3.3.4 Blade tip detection ................................ 48
3.3.5 Vortex detection in PIV flow fields ................ 49
3.3.6 Evaluation of axial and swirl velocity ............. 52
3.3.7 Individual averaging ............................... 54
3.3.8 PIV measurement accuracy ........................... 54
3.3.9 Evaluation of BOS raw data ......................... 60
3.3.10 Reconstruction of vortex positions from BOS data ... 61
3.3.11 Density reconstruction ............................. 63
4 STAR results and discussion ............................... 67
4.1 Baseline case ............................................. 67
4.1.1 Baseline case: time-resolved velocity and
vorticity fields ................................... 67
4.1.2 Baseline case: vortex trajectories from PIV ........ 70
4.1.3 Baseline case: vortex trajectories from BOS ........ 72
4.1.4 Baseline case: vortex characterization from PIV .... 73
4.1.5 Baseline case: vortex characterization from BOS .... 74
4.2 Thrust variation case ..................................... 75
4.2.1 Thrust variation case: vortex trajectories from
PIV ................................................ 76
4.2.2 Thrust variation case: vortex strength from PIV .... 77
4.2.3 Thrust variation case: vortex strength from BOS .... 78
4.2.4 Summary ............................................ 79
4.3 Active twist actuation .................................... 80
4.3.1 Active twist actuation: blade deformation .......... 80
4.3.2 Active twist actuation: vortex trajectories ........ 83
4.3.3 Active twist actuation: vortex strength ............ 88
4.3.4 Active twist actuation: swirl velocity and core
size for 3/rev ..................................... 92
4.3.5 Active twist actuation: 3D vorticity fields for
2/rev and 3/rev .................................... 93
4.4 Summary of the STAR hover test ............................ 96
5 Localization of blade tip vortices on a full-scale
helicopter ................................................ 99
5.1 Measurement techniques ................................... 100
5.1.1 Large-scale vortex visualization techniques ....... 100
5.1.2 Natural background BOS ............................ 101
5.1.3 Reference-free BOS ................................ 103
5.2 Preliminary flight tests ................................. 105
5.2.1 Reference-free BOS hover test in ground effect .... 105
5.2.2 Reference-free BOS maneuvering flight test ........ 107
5.2.3 Airborne reference-free BOS inflight test ......... 108
5.2.4 Concluding remarks about the preliminary flight
tests ............................................. 110
5.3 3D BOS flight test ....................................... 110
5.3.1 Measurement setup ................................. 110
5.3.2 BO 105 helicopter ................................. 113
5.3.3 Flight conditions ................................. 113
5.3.4 Concluding remarks about the 3D BOS flight test
setup ............................................. 114
5.4 BOS data analysis ........................................ 114
5.4.1 Vortex visualization using the BOS technique ...... 115
5.4.2 Discretization and pairing of vortex segments ..... 116
5.4.3 Iterative camera calibration ...................... 117
5.4.4 Epipolar geometry ................................. 118
5.4.5 3D reconstruction of vortex positions ............. 120
5.4.6 Numerical comparison .............................. 121
5.4.7 Concluding remarks about the BOS data analysis .... 121
5.5 Results of the full-scale vortex reconstruction .......... 122
5.5.1 Multi-camera vortex visualization ................. 122
5.5.2 Full-scale vortex reconstruction .................. 123
5.5.3 Summary of the flight tests ....................... 126
6 Conclusions and outlook .................................. 127
6.1 Active twist investigation on a large-scale model rotor .. 127
6.2 Three-dimensional vortex localization on helicopters ..... 129
6.3 Outlook .................................................. 130
References .................................................... 132
Curriculum Vitae .............................................. 145
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